Studies have revealed that there is a class of automotive accidents causing injuries associated with airbag deployment and with the nature and position of the vehicle occupant, particularly with respect to airbags deployed adjacent to seats occupied by children or infants in car seats. Automotive occupancy sensor (AOS) systems used in conjunction with airbag deployment systems (ADS) have been developed to regulate the deployment of the airbag, i.e., to determine if deployment is to be aborted, deferred, modified or otherwise controlled in response to the occupancy state of the adjacent vehicle interior. For background on AOS systems see Corrado, et al., U.S. Pat. No. 5,482,314 issued Jan. 9, 1996, and U.S. Pat. No. 5,890,085 which issued Mar. 30, 1999, which patents are hereby incorporated by reference.
AOS utilize various types of sensors which produce signals which provide information relating to occupancy state. Typically electrostatic ultrasound transducers are included in AOS systems as active sensors whereby echoes of ultrasonic signals transmitted by the transducer are detected by the transducer when reflected back from the vehicle interior and occupants.
The electrostatic transducer response is highly temperature sensitive, and vehicle interiors have a large potential temperature range during operation, particularly when driving has just started after some period of non-use in hot or cold weather, and environmental controls such as heaters or air conditioners have not yet moderated extremes of heat or cold of the vehicle interior. The amplitude of the received ultrasound signal (echo signal) using an electrostatic ultrasound sensor may change by more than 200% over a temperature range of -40.degree. C. (-40.degree. F.) to +80.degree. C. (+176.degree. F.). This change has degrading impacts on a typical AOS occupancy classification algorithm.
One solution which has been proposed is to develop a classification algorithm that is robust against changes in signal amplitude. This approach has been tried with limited success, because in an ultrasound signal for occupant classification, a large amount of the distinguishing information is amplitude related, i.e., a lot of information needed for occupancy classification is present in the amplitude of the ultrasonic echo. Amplitude changes due to temperature rather than due to changes in the state of the occupants or objects being imaged could result in false classifications. It is thus important to keep amplitude variations due to environmental effects small, so that any amplitude variations seen in the echo signal are, reliably, only due to changes in the state of objects or occupants present in the vehicle.
What is needed is a mechanism and method that compensates or normalizes the ultrasound signal with respect to temperature variation such that it appears to the classification algorithm to have been collected at or near a reference temperature at which the classification algorithm is optimized, preferably near room temperature.